JP4085490B2 - Synthetic quartz glass for optical members and manufacturing method thereof - Google Patents

Description

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【表２】

【００３１】
【表３】

【００３２】
【発明の効果】
本発明によれば、エキシマレーザなどの紫外線照射によってもＥ’センタによる吸収帯の生成が少なく、耐紫外線性に優れた光学部材用合成石英ガラスが得られる。また、本発明の製造方法によれば、エキシマレーザなどの紫外線照射によってもＥ’センタによる吸収帯の生成が少なく、耐紫外線性に優れた光学部材用合成石英ガラスを、厳しい条件や大がかりな装置を必要とすることなく、生産性よく、簡便に得ることができる。
【図面の簡単な説明】
【図１】ＫｒＦエキシマレーザ照射前後の吸収係数の変化を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a synthetic quartz glass for optical members used in an apparatus using ultraviolet rays having a wavelength of 400 nm or less as a light source, and a method for producing the same. Specifically, excimer laser (XeCl: wavelength 308 nm, KrF: wavelength 248 nm, ArF: wavelength 193 nm), F ₂ laser (wavelength 157 nm), low-pressure mercury lamp (wavelength 185 nm), excimer lamp (XeXe: wavelength 172 nm), deuterium lamp The present invention relates to a synthetic quartz glass for optical members used as optical parts such as lenses, prisms, window materials and the like used for light from an ultraviolet region to a vacuum ultraviolet region emitted from a light source such as (wavelength 170 to 400 nm) and a method for producing the same.
[0002]
[Prior art]
Synthetic quartz glass is a transparent material over a wide wavelength range from the near infrared region to the vacuum ultraviolet region, has an extremely small coefficient of thermal expansion and excellent dimensional stability, and contains almost no metal impurities. It has features such as high purity. Therefore, synthetic quartz glass has been mainly used as an optical member of an optical device using conventional g-line and i-line as a light source.
[0003]
In recent years, with the high integration of LSIs, a fine drawing technique with a shorter line width has been required in the optical lithography technique for drawing an integrated circuit pattern on a wafer. Is being promoted. For example, the light source of a stepper for lithography is advanced from the conventional g-line (wavelength 436 nm) and i-line (wavelength 365 nm), KrF excimer laser (wavelength 248 nm), ArF excimer laser (wavelength 193 nm), and F ₂ laser (wavelength). 157 nm) is about to be used.
[0004]
The low-pressure mercury lamp (wavelength 185 nm) and excimer lamp (XeXe: wavelength 172 nm) are used in 1) equipment such as photo-CVD, 2) ashing and etching equipment for silicon wafers, or 3) ozone generators. Yes. Development is progressing to be applied to photolithography technology in the future. Synthetic quartz glass must also be used for optical components used in gas sealed tubes such as low-pressure mercury lamps, excimer lamps, deuterium lamps, or optical devices using the aforementioned short-wavelength light sources.
[0005]
Synthetic quartz glass used in these optical systems is required to have light transmittance at wavelengths ranging from the ultraviolet region to the vacuum ultraviolet region, and has high light resistance at the wavelength used (the transmittance does not decrease after light irradiation). ) Is required.
[0006]
In conventional synthetic silica glass, when high energy light emitted from a light source such as a KrF excimer laser or an ArF excimer laser is irradiated, a new absorption band is generated in the ultraviolet region, which causes a problem in use as an optical member. That is, when ultraviolet rays are irradiated for a long time, an absorption band of about 215 nm called a so-called E ′ center (≡Si ·) occurs.
[0007]
The absorption band due to the E ′ center is considered to be caused by defects due to unstable structures such as a three-membered ring structure and a four-membered ring structure in synthetic quartz glass. When an absorption band of approximately 215 nm is generated, there are problems such as a decrease in transmittance, an increase in absolute refractive index, a change in refractive index distribution, and fluorescence.
[0008]
In order to improve UV resistance, it is important to reduce defects due to unstable structures such as three-membered ring structures and four-membered ring structures. Various methods have been studied for this purpose. For example, after re-melting synthetic quartz glass at a high temperature of about 1600 ° C in a high-pressure rare gas-containing atmosphere of 500 kgf / cm ² or more, the temperature is lowered to 500 ° C in the same atmosphere, thereby reducing unstable structures. Have been proposed (JP-A-4-164834, JP-A-5-43267). However, these proposed methods require a high pressure and require a large apparatus. Further, since the processing is performed at a high temperature, impurities are mixed in the synthetic quartz glass, and the purity of the synthetic quartz glass may be lowered. It was.
[0009]
[Problems to be solved by the invention]
The present invention has been made to solve the above problems, and an object of the present invention is to provide a synthetic quartz glass for an optical member that is less likely to generate an absorption band due to an E ′ center even when irradiated with ultraviolet rays and has excellent ultraviolet resistance. Another object of the present invention is to provide a production method for easily obtaining such synthetic quartz glass for optical members with high productivity.
[0010]
[Means for Solving the Problems]
The present invention is a method for producing synthetic quartz glass for optical members used for light in the ultraviolet region to the vacuum ultraviolet region, and a temperature T _{1 at} which the viscosity of synthetic quartz glass having an OH group concentration of 100 ppm or less becomes 10 ^14.5 poise. Held at a temperature equal to or lower than (° C.) and equal to or higher than a temperature T ₂ (° C.) indicated by T ₁ -200 (° C.) for 80 hours or longer, and a scattering peak intensity I ₁ of 495 cm ⁻¹ in a laser Raman spectrum. , Scattering peak intensity I _{2 at} 605 cm ⁻¹ , And a scattering peak intensity I _{0 at} 440 cm ⁻¹ But I ₁ / I ₀ ≦ 0.57 and I ₂ / I ₀ There is provided a method for producing a synthetic quartz glass for an optical member, comprising a step of producing a synthetic quartz glass satisfying a relationship of ≦ 0.14 .
[0011]
The present inventors have found that the unstable structure in the synthetic quartz glass can be reduced by holding the synthetic quartz glass at a temperature in a certain range for a predetermined time.
T ₁ is a so-called strain point, which depends on the content of OH, F, etc. in the synthetic quartz glass, or the content of transition metals, chlorine, etc. contained as impurities. ˜1200 ° C.
[0012]
When held at a temperature exceeding T ₁ , the unstable structure in the synthetic quartz glass is not reduced, and when held at a temperature lower than T ₂ , it is necessary to reduce the unstable structure in the synthetic quartz glass. The time is very long, 1000 hours or more, which is not efficient.
[0013]
The reason why such an optimum temperature range exists is not clear, but it is closely related to the viscosity of the synthetic quartz glass. If the temperature is too high, the network structure of the synthetic quartz glass becomes a fluid state and is maintained at a high temperature in the fluid state. This is probably because the unstable structure is not reduced. If the temperature is too low, the network structure of synthetic quartz glass is fixed, and it is considered that it takes a long time to reduce the unstable structure.
[0014]
In the present invention, the time for holding at the temperature within the above range is preferably 10 hours or more, particularly preferably 80 hours or more. In practice, 800 hours or less is preferable. About the atmosphere at the time of holding | maintenance, 1 or more types of gas chosen from the group which consists of hydrogen gas, oxygen gas, nitrogen gas, and a noble gas is employable. In particular, an inert gas such as nitrogen gas or a rare gas (for example, helium) is preferable from the viewpoint of safety of equipment and suppression of defect generation of synthetic quartz glass.
[0015]
In the present invention, from the viewpoint of improving the optical homogeneity of the obtained synthetic quartz glass and suppressing the generation of distortion, it is indicated by T ₁ -500 (° C.) after the step of holding at the temperature of T ₂ or higher. It is preferable that the method further includes a step of lowering the temperature to a temperature equal to or lower than a temperature T ₃ (° C.) at a temperature lowering rate of 50 ° C./hr or lower.
[0016]
The present invention is also a synthetic quartz glass for optical members used for light from the ultraviolet region to the vacuum ultraviolet region, having an OH group concentration of 100 ppm or less, and a scattering peak intensity I ₁ of 495 cm ⁻¹ in a laser Raman spectrum. , 60 5 cm ⁻¹ scattering peak intensity I ₂ and 440 cm ⁻¹ scattering peak intensity I ₀ satisfy the relationship of I ₁ / I ₀ ≦ 0.57 and I ₂ / I ₀ ≦ 0.14 A synthetic quartz glass for an optical member is provided.
[0017]
The 495 cm ⁻¹ scattering peak and the 605 cm ⁻¹ scattering peak are peaks due to a three-membered ring structure and a four-membered ring structure, respectively, and the 440 cm ⁻¹ scattering peak is a peak due to fundamental vibration between silicon and oxygen, and I ₁ / I ₀ and I ₂ / I ₀ represent the relative concentrations of the three-membered ring structure and the four-membered ring structure in the synthetic quartz glass for optical members. Note that the above-described 495 cm ⁻¹ , 605 cm ^−1, and 440 cm ⁻¹ may be slightly shifted depending on a measuring device, a measurement sample, or the like.
[0018]
From the viewpoint of reducing the fluorescence intensity, the OH group concentration is preferably 400 ppm (meaning weight ppm, the same shall apply hereinafter) or less, particularly preferably 100 ppm or less. Further, from the viewpoint of reducing the fluorescence intensity, the hydrogen molecule concentration is preferably 5 × 10 ¹⁶ molecules / cm ³ or more.
The synthetic quartz glass for optical members of the present invention can be obtained, for example, by the production method described above.
[0019]
【Example】
SiO ₂ fine particles formed by hydrolyzing SiCl ₄ in an oxyhydrogen flame were deposited on a substrate to synthesize a porous quartz glass body having a diameter of 500 mmφ and a length of 700 mm. This porous quartz glass body is placed in an electric furnace capable of controlling the atmosphere, heated to 1450 ° C. under a helium gas atmosphere containing water vapor under normal pressure, and kept at this temperature for 3 hours to form a transparent glass. A synthetic quartz glass having a diameter of 250 mm and a length of 450 mm was obtained.
[0020]
Here, the water vapor partial pressure in the atmosphere during vitrification is adjusted to control the OH group content, and the strain points T ₁ shown in Table 1 are 4 types of 1110 ° C., 1090 ° C., 1070 ° C., and 950 ° C. (T ₂ is 910 ° C., 890 ° C., 870 ° C., and 750 ° C., respectively, and T ₃ is 610 ° C., 590 ° C., 570 ° C., and 450 ° C., respectively). The OH group concentrations of the synthetic quartz glass having strain points T ₁ of 1110 ° C., 1090 ° C., 1070 ° C., and 950 ° C. are 33 ppm, 120 ppm, 230 ppm, and 970 ppm, respectively.
[0021]
Next, a synthetic quartz glass having a size of 200 mmφ × 30 mmt was cut out from a synthetic quartz glass having a diameter of 250 mm and a length of 450 mm, and heat treatment was performed under the heat treatment conditions shown in Table 1 except that the atmospheric conditions were fixed at 1 atm of helium gas. Slow cooling after the heat treatment was performed under the slow cooling conditions shown. In addition, furnace cooling is the meaning which stood to cool in a furnace.
[0022]
After the above slow cooling, it was further kept at 500 ° C. for 250 hours in an atmosphere containing hydrogen gas to dope hydrogen. Examples 1 to 15 have a hydrogen gas 100% atmosphere and 10 atmospheres (absolute pressure), Example 16 has a hydrogen gas 100% atmosphere and 1 atmosphere (absolute pressure), and Example 17 has a hydrogen gas / He of 10/90 ( Each was doped with hydrogen in an atmosphere (volume ratio) and 1 atmosphere (absolute pressure). Table 1 shows the hydrogen molecule concentration of the obtained synthetic quartz glass. There was no change in the OH group concentration.
[0023]
The obtained synthetic quartz glass was evaluated as follows. Examples 1-3, Examples 7-10, and Examples 14-17 correspond to Examples, and Examples 4-6 and Examples 11-13 correspond to Comparative Examples.
[0024]
(Evaluation 1)
Raman spectroscopy (Jobin Ybon made Ramonor T64000, excitation light source: argon ion laser (wavelength 514.5 nm)) performed, and the scattering peak intensity I ₂ of the scattering peak intensity of 495cm ^-1 in the laser Raman spectrum I ₁ and 605 cm ^-1, Intensity ratios I ₁ / I ₀ and I ₂ / I ₀ with respect to the intensity I ₀ of the scattering peak at 440 cm ⁻¹ were determined. The smaller the intensity ratio I ₁ / I ₀ and the intensity ratio I ₂ / I ₀ , the better. The evaluation results are shown in Table 2.
Each scattering peak intensity I _1, I _2, of determining the I ₀ is as follows.
[0025]
Perform curve fitting by a single Lorentzian respectively scattering peak of scattering peak and 605 cm ^-1 of 495cm ^-1, the coefficients of the functions performed approximated as least square error is minimum between the actual spectrum Were determined.
The synthesis of the Gaussian function of the three for scattering peak of 440 cm ^-1, also with respect to the remainder (baseline), excluding the 495cm ^-1 scattering peak and 605 cm ^-1 scattering peak and 440 cm ^-1 scattering peak Curve fitting was performed using a quadratic function, and approximation was performed so that the least square error with the actual spectrum was minimized, and the coefficient of each function was determined.
The intensity | strength of each scattering peak was calculated | required using the function calculated | required by the above.
[0026]
(Evaluation 2)
Using a birefringence meter (ADR-150LC, manufactured by Oak Manufacturing Co., Ltd.), the amount of birefringence in the plane of 200 mmφ is measured, and when the amount of birefringence in the plane of 200 mmφ is 10 nm / cm or less, OK is over 10 nm / cm. In some cases, it was judged as NG. The amount of birefringence is an index for measuring the magnitude of distortion. The smaller the value, the better the distortion. The evaluation results are shown in Table 2.
[0027]
(Evaluation 3)
For the synthetic quartz glass of Examples 1 and 6, a sample having a size of 30 mmφ × 10 mmt was further cut out from a size of 200 mmφ × 30 mmt, and light from a KrF excimer laser (LPX-120i manufactured by Lambda Fijik) was used with an energy density of 100 mJ / cm ² / The sample was irradiated under conditions of pulse, frequency 200 Hz, 3 × 10 ⁶ shots. The transmittance at 190 to 250 nm before and after irradiation was measured, and the change in absorption coefficient due to irradiation was calculated from the transmittance. The smaller the change in absorption coefficient, the better the UV resistance. The change in absorption coefficient before and after irradiation is shown in FIG. In FIG. 1, 2.0E-03 means 2.0 × 10 ⁻³ , and so on.
[0028]
(Evaluation 4)
For the quartz glasses of Examples 1, 8, and 14-17, a sample having a size of 30 mmφ × 10 mmt was cut out in the same manner as in Evaluation 3, and light from the KrF excimer laser used in Evaluation 3 was energy density 100 mJ / cm ² / pulse, frequency 200 Hz. The sample was irradiated under conditions of 1 × 10 ⁶ shots. When the light from the KrF excimer laser is irradiated, the fluorescence intensity L _{650 at 650} nm and the scattered light intensity S ₂₄₈ at ₂₄₈ nm are measured using a fiber light guide type spectrophotometer, respectively, and the ratio L ₆₅₀ / S _{248 of} both is measured. Was evaluated for fluorescence intensity. The evaluation results are shown in Table 3.
[0029]
[Table 1]

[0030]
[Table 2]

[0031]
[Table 3]

[0032]
【The invention's effect】
According to the present invention, a synthetic quartz glass for an optical member that is less likely to generate an absorption band due to an E ′ center even when irradiated with ultraviolet rays such as an excimer laser and has excellent ultraviolet resistance can be obtained. In addition, according to the manufacturing method of the present invention, the synthetic quartz glass for optical members, which is less likely to generate an absorption band by the E ′ center even when irradiated with ultraviolet rays such as an excimer laser and has excellent ultraviolet resistance, Can be easily obtained with good productivity.
[Brief description of the drawings]
FIG. 1 is a graph showing a change in absorption coefficient before and after irradiation with a KrF excimer laser.

Claims (6)

A method for producing synthetic quartz glass for optical members used for light from the ultraviolet region to the vacuum ultraviolet region, wherein the synthetic quartz glass having an OH group concentration of 100 ppm or less has a temperature T ₁ (° C.) at which the viscosity becomes 10 ^14.5 poise. The scattering peak intensity I _{1 at} 495 cm ⁻¹ in the laser Raman spectrum is maintained for 80 hours or more at a temperature equal to or higher than the temperature T ₂ (° C.) indicated by T ₁ -200 (° C.). , Scattering peak intensity I _{2 at} 605 cm ⁻¹ , And a scattering peak intensity I _{0 at} 440 cm ⁻¹ But I ₁ / I ₀ ≦ 0.57 and I ₂ / I ₀ The manufacturing method of the synthetic quartz glass for optical members characterized by including the process made into the synthetic quartz glass which satisfy | fills the relationship of <= 0.14 . The method further includes a step of lowering the temperature at a temperature lowering rate of 50 ° C./hr or less to a temperature of T ₃ (° C.) or less indicated by T ₁ -500 (° C.) after the step of holding at T ₂ or higher. 2. A method for producing a synthetic quartz glass for an optical member according to 1. The method for producing a synthetic quartz glass for an optical member according to claim 1 or 2, wherein the synthetic quartz glass has an OH group concentration of 33 pm or less. An optical member for a synthetic quartz glass used to light from ultraviolet region to a vacuum ultraviolet region, OH group concentration is at 100ppm or less, the scattering peak intensity of 495cm ^-1 in the laser Raman spectrum I _1, 60 5 cm ^-1 An optical member, wherein the scattering peak intensity I ₂ and the scattering peak intensity I ₀ of 440 cm ⁻¹ satisfy the relationship of I ₁ / I ₀ ≦ 0.57 and I ₂ / I ₀ ≦ 0.14 Synthetic quartz glass for use. 5. The synthetic quartz glass for optical members according to claim 4 , wherein the concentration of hydrogen molecules is 5 × 10 ¹⁶ molecules / cm ³ or more. The synthetic quartz glass for optical members according to claim 4 or 5, wherein the synthetic quartz glass has an OH group concentration of 33 pm or less.

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